The present invention relates to the preparation of glyphosate, N-phosphonomethyl glycine, by the catalyzed oxidation of N-phosphonomethyliminodiacetic acid.
The synthesis of glyphosate (PMG) by oxidation of N-phosphonomethyliminodiacetic acid (PMIDA) over a catalyst appears, at least on the surface, to be relatively straightforward. However, this reaction can be prone to vagaries that are particularly vexing to chemists. Part of the complexity lies in the identification, selection, and preparation of a suitable catalyst, and part of the complexity lies in the competing side reactions, which generate by-products. In addition, when a product such as PMG is selected for commercialization, the complexity of these problems and the number of potential solutions to the problems are multiplied. Well over 250,000 metric tons of glyphosate are sold annually. The production costs, including costs of starting materials, time, energy requirements, purification, waste management, and of course product yield, take on an even greater consideration.
While there have been significant advances in catalyst research, much about the specific catalysts, their properties, and their roles in a particular reaction remains a mystery. For this particular reaction, a wide variety of catalysts have been investigated. Each catalyst appears to have at least some advantages and, notably, significant disadvantages. For example, a carbon supported noble metal catalyst useful for oxidizing PMIDA is disclosed in U.S. Pat. No. 3,950,402. These catalysts tend to leach noble metals into the reaction solution under the reaction conditions. Each of U.S. Pat. Nos. 4,147,719, 5,179,228, WO 01/92272, and U.S. Pat. No. 6,337,298 discloses the use of different carbon supported noble metal catalysts that were treated to reduce noble metal leaching. However, the carbon supported noble metal catalysts are costly to prepare and the spent catalysts are expensive to recycle and treat. Furthermore, the carbon supported noble metal catalysts still generate side products in the oxidation reaction. Use of an activated carbon catalyst without any noble metals (or co-catalysts) in the PMIDA oxidation reaction is disclosed by Hershman in U.S. Pat. No. 3,969,398. The carbon catalyst produced relatively high yields of PMG in a batch reactor. However, the yields were lower in a continuous flow fixed bed reactor.
There continues to be research for improved catalysts and for methods of evaluating the different catalysts. For example, Cho in U.S. Pat. No. 4,624,937 and U.S. Pat. No. 4,696,722 disclosed carbon catalysts that were prepared by removing oxides from the carbon surface at high temperatures to form a highly reduced and a more highly activated carbon catalyst. Different methods have been used to evaluate the activity of a particular catalyst. For example, the decomposition of hydrogen peroxide by activated carbons was described in the literature at least as early as 1966 [R. N. Smith, et al., Trans. Faraday Soc., 62, 2553-2565 (1996)]. Activated carbons can be characterized by measuring the rate of hydrogen peroxide decomposition over these materials [K. H. Radeke, et al., Acta Hydrochim. Hydrobiol., 17, 315-319 (1989)]. Hayden et al. in U.S. Pat. No. 5,470,748 describes a specific method for measuring the catalytic activity of pulverized carbonaceous chars by measuring the time required for each to decompose a given quantity of hydrogen peroxide.
This peroxide procedure was subsequently used by Hayden et al. in U.S. Pat. No. 5,962,729 and by Farmer et al. in U.S. Pat. No. 5,942,643 to evaluate carbonaceous char useful for the oxidation of PMIDA to PMG. These references conclude that a highly reduced, fine particulate (particles smaller than 325 mesh; <44 μm) carbonaceous char exhibits high catalytic activity. The highly reduced carbonaceous char was used in a batch process to oxidize PMIDA to PMG as well as to produce carbon dioxide, presumably by oxidizing the formaldehyde and formic acid by-products from the oxidation reactions. However, no yields of PMG are disclosed. Furthermore, the reaction selectivity for PMG over the by-products such as (aminomethyl)phosphonic acid (AMPA), N-methyl-N-(phosphonomethyl)glycine (MePMG), or (methylaminomethyl)phosphonic acid (MAMPA) is not disclosed.
In WO 01/92272 various catalysts and processes for the production of PMG are described. The processes include oxidizing PMIDA over a carbon supported catalyst in a series of back-mixed zone reactors, fluidized bed reactors and/or fixed bed reactors. Different types of catalysts were specifically selected for use in the different reactors. For example, in a fixed bed reactor the carbon catalysts were selected to be highly active, particulate carbon-supported catalysts, extruded carbon supported catalysts, or non-conventional fixed bed catalyst supports, such as a monolithic screens or honeycomb supports. Additionally, the carbon supports can include one or more noble metals and optionally other metals deposited on the carbon. The highly active catalysts were used to further oxidize the formaldehyde and formic acid by-products to carbon dioxide and water. This reference did not provide any results demonstrating that high yields of PMG could be obtained in a fixed bed reactor. Furthermore, as mentioned above, the highly active catalysts and the noble metal carbon supported catalysts are expensive to produce and tend to leach noble metals into the reaction solution unless additional precautions are observed.
The reaction conditions for the oxidation of PMIDA can also greatly influence the yield of PMG and mixture of by-products. The yield of PMG can be low because of incomplete conversion of the PMIDA. Alternatively, if PMIDA is completely consumed, the yield of PMG can be low because of competing side reactions that produce a number of by-products such as (aminomethyl)phosphonic acid (AMPA) or formaldehyde and formic acid, which in turn are thought to be involved in the methylation of PMG and AMPA to yield N-methyl-N-(phosphonomethyl)glycine (MePMG) and (methylaminomethyl)phosphonic acid (MAMPA).
In some aspects PMG can be considered as an intermediate in a series of reactions from PMIDA to AMPA or the methylated product MAMPA. These competing series of reactions can lower the yield of the desired PMG product as well as make purification procedures and waste treatment processes more time consuming and costly. Again, the economic and environmental considerations for the large scale, commercially viable production of PMG require that the reaction provide a single, readily isolable product.
When a large scale production of a product is contemplated, it is typically considered that a continuous process provides economical advantages over a batch process. Using a fixed bed reactor in the continuous process can provide additional advantages. The total reactor volume for a fixed bed process is usually reduced relative to that required for a batch or continuous slurry process. In addition to the corresponding reduction in capital expense, a process using a fixed bed reactor is less labor intensive. Catalyst expenses will also be less for a continuous, steady-state fixed bed process as opposed to a slurry process where catalyst filtration and recycle are required.
In light of the above described problems, there is a continuing need for advancements in the relevant field, including improved synthetic methods to produce PMG using a environmentally sound process, and considering efficiency and economics to provide quantities of PMG suitable for commercialization via large scale production. The present invention is such an advancement and provides a wide variety of benefits and advantages.